Liquid crystal displays have been used in various embodiments such as watches and desk calculators because of their flatness, lightness, and low electric power consumption. With the advancement of integrated circuits (IC), liquid crystal displays have been increasing the display size and extending their use in computers, liquid crystal TV sets, etc. in place of conventional cathode-ray tubes. However, nematic liquid crystals which have conventionally been used have a slow response time of from 10 to 50 milliseconds and undergo a reduction in display contrast according as the number of pixels increases.
In the state-of-the-art liquid crystal displays, the above-described disadvantage is coped with by fitting each pixel with a thin film transistor (TFT) to achieve so-called active matrix driving or by increasing the angle of twist of liquid crystal molecules sandwiched between a pair of substrates to 220.degree. to 270.degree. (super-twisted nematic: STN).
Mounting of TFT according to the former means not only entails very high cost but has a poor yield, resulting in an increased production cost. Cost reduction by introducing a large-scale production line having been studied, there is a limit due to essential involvement of many production steps. Further, ever since the appearance of high-definition televisions (HDTV), there has been an increasing demand of liquid crystal displays making a high-content display. In nature of TFT and nematic liquid crystals, it is nevertheless considered very difficult to increase display density.
On the other hand, although the STN mode exhibits an increased contrast ratio, it has a slower response time of from 100 to 200 milliseconds and is thus limited in application.
It has therefore been keenly demanded to develop a liquid crystal element which achieves high-density displaying at a high rate of response. Ferroelectric liquid crystal display elements form the nucleus of such expectations. Ever since the report of N. A. Clark, et al. on surface-stabilized ferroelectric liquid crystal devices (SSFLD) (refer to N. A. Clark, et al., Appl. Phys. Lett., Vol. 36, p. 899 (1980), extensive studies have been directed to ferroelectric liquid crystals with the attention on their fast response time. However, ferroelectric liquid crystal display elements have not yet been put into practical use due to problems of response rate, molecular orientation, etc. still remaining unsolved. For example, the molecular orientation of ferroelectric liquid crystals proved more complicated than suggested by Clark, et al. That is, the director of liquid crystal molecules is apt to be twisted in smectic layers, under which a high contrast cannot be obtained. Further, the layers have been believed to be aligned upright and perpendicular to the upper and lower substrates (bookshelf structure) but, in fact, were found to have a bent structure (chevron structure). As a result, zigzag defects appear to reduce a contrast ratio. As an approach to solutions to these orientation problems, improved orientation methods have recently been proposed, such as use of an oblique SiO-deposited technique.
With respect to response time, it was believed in the early stage of studies that ferroelectric liquid crystal elements have a response in several microseconds. In fact, however, the highest of the so far reached response time is only several tens of microseconds. That is, in ferroelectric liquid crystal elements, since a response time of one pixel decides a refreshing time of a display unlike nematic liquid crystal elements, advantages of ferroelectric liquid crystals cannot be made full use of unless a fast response time of from 20 to 30 microseconds or less is reached. A response time is considered dependent on spontaneous polarization and rotational viscosity of liquid crystal materials and intensity of the applied electric field. Considering a limit of voltage which can be applied in practice with IC, a substantial improvement in response time must be realized through optimization of rotational viscosity and spontaneous polarization of liquid crystal materials. Under the present situation, sufficiently fast response time has not yet been obtained.
Hence, a search has been made preponderantly for compounds having large spontaneous polarization and liquid crystal compounds having low rotational viscosity. However, compounds exhibiting high spontaneous polarization generally have high viscosity, and few compounds satisfying both of these requirements have been discovered.
In general, a ferroelectric liquid crystal material comprises an achiral base liquid crystal composition showing a smectic C phase (Sc phase) to which an optically active compound called chiral dopants are added to form a ferroelectric liquid crystal composition. This is because, for one thing, performance requirements cannot be satisfied by a single material and, for another thing, it is aimed at to allot each of required performance properties to each compound so as to make a mixed system as simple as possible taking advantage of the fact that various physical properties including orientation vary depending on the structure of liquid crystal compounds used. Namely, in order to satisfy many physical properties required for ferroelectric liquid crystals, it is advantageous to divide the functions among components as simply as possible. In many cases, phenylpyrimidine type liquid crystal compounds having advantageous viscosity properties are utilized as an achiral base. In actual use, however, properties of the resulting ferroelectric composition, such as viscosity and response time, greatly vary depending on the properties of optically active compounds added thereto. Accordingly, an optically active compound to be added is required to exhibit moderate spontaneous polarization and low viscosity to provide a liquid crystal composition exhibiting a fast response time while giving no adverse influence on the performance of the achiral base, such as a temperature range.
Further, in order to obtain satisfactory orientation, ferroelectric liquid crystals are often required to have a smectic A phase in which orientation can be effected with relative ease and, if possible, a nematic phase in a higher temperature range. This being the case, when a chiral dopant is added to an achiral base having a phase sequence of isotropic, nematic, S.sub.A and Sc phase, the N and Sc phases become a chiral nematic phase (N.sup.* phase) and a chiral smectic phase (Sc.sup.* phase), respectively, in which a helical structure is induced as shown in FIGS. 1 and 3.
In order that the ferroelectric liquid crystals show satisfactory orientation and satisfactory bistability for use as a ferroelectric liquid crystal optoelectronic element, the N.sup.* phase and Sc.sup.* phase should have a helical pitch several times longer than the cell thickness. To achieve this, addition of only one kind of a chiral dopant is not sufficient, and it is necessary to use a chiral dopant in combination with an optically active compound showing an opposite helical sense. Besides, every optically active compound has its own direction, positive or negative, in spontaneous polarization. The helical sense of the N.sup.* and Sc.sup.* phases and the direction of spontaneous polarization are decided by the structure of the optically active compound without being correlated to each other. Therefore, mixing of optically active compounds makes the problem more complicated.
Hence, it is required to control the helix in the N.sup.* and Sc.sup.* phases with optically active compounds of as small kinds as possible while obtaining effective spontaneous polarization.
The problem to be considered here is temperature dependence of the helical pitch in the N.sup.* and Sc.sup.* phases induced by a chiral dopant. As mentioned above, the helical pitch is required not only to be sufficiently longer over the cell thickness but to have small temperature dependence. Even with a sufficiently long helical pitch, large temperature dependence results in great variation of orientation.
The importance of temperature dependence of a helical pitch also applies to a chiral dopant to be added to nematic liquid crystals which are used in twisted nematic (TN) and also STN mode display elements. For example, if a chiral dopant to be used shows high positive dependence on temperature (i.e., the pitch is broadened with an increase in temperature), it must be mixed with a chiral dopant having an opposite tendency to offset the temperature dependence, which makes the chiral dopant mixing system more complicated.
Thus, optically active compounds and compositions thereof for use in ferroelectric liquid crystal elements are demanded to satisfy a variety of performance properties. Accordingly, an optically active compound which has a fast response time, which does not have great influences on the temperature range of an achiral base, and whose helical pitch has small temperature dependence is very important for advancement of ferroelectric liquid crystals. For the time being, only a few of such compounds have been reported.
Further, chiral dopants currently used for nematic liquid crystals comprise a mixture of at least 4 kinds of optically active compounds for the purpose of the control of helical pitch in the N.sup.* phase and reducing the temperature dependence of the helical pitch. Not a few of these optically active compounds exhibit no liquid crystalline properties and, when added to a nematic liquid crystal, cause a drop of the nematic phase-isotropic phase transition temperature.